Mayor Bloomberg would win the Democratic primary for mayor if he were allowed to enter it, according to a poll released yesterday.

Bloomberg, a Republican, would demolish Democratic front-runner Fernando Ferrer with 42 percent of the vote – compared to Ferrer’s 23 percent, the Marist College poll found.

In that fictional contest, Rep. Anthony Weiner would grab just 10 percent of the vote, and City Council Speaker Gifford Miller and Manhattan Borough President C. Virginia Fields would drop into single digits.

But the mayor, a former Democrat, declined to gloat.

“I’m just going to focus on running the city and not get involved,” he said during a visit to a Red Cross station on the Upper West Side. “Today’s not a good day for politics.”

The poll found nearly a quarter of Democratic voters still not sure which of the four Democratic candidates to support – an unusual situation, since the primary is Tuesday.

Given a choice of Democrats, former Bronx Borough President Ferrer kept his lead, but with only 32 percent of the vote – well short of the 40 percent needed to avert a run-off against the second-place finisher.

Weiner inched up to second with 17 percent; Miller was right behind at 15 percent; and Fields faded to last with 13 percent.

“It’s all very fluid,” said pollster Lee Miringoff. “This is a wide-open primary. Ferrer is leading, but it’s not clear if he makes the 40 percent. It’s not clear who comes in second.”

Thirteen percent of the 735 registered Democrats questioned Aug. 20-Sept. 2 said they couldn’t state a preference under any circumstance at this point.